Also look for any bent pins on the port where you plug in the controller. Make sure there's no debris in there either, even a metal shaving could cause a short in the exposed ports.
That said, I know you said it's not the controller, but the controllers are almost always the cause of this issue. In your case, it sounds like a short somewhere, but others should be opening the controller and checking for a broken or twisted plastic keypress ring on the controller, or a controller switch whose tape has worn off, allowing the dome to touch the base wire. Simply re-taping with cellophane tape is usually good enough to fix this.